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What is Causing the Recent Increases in NO3 in Loch Vale Surface Waters?
J.S. Baron*
USGS, Natural Resource Ecology Laboratory, CSU, Fort Collins, CO
T.R.Schmidt
USGS, Mineral Resources Team, Denver CO
A. Krcmarik
USGS, Fort Collins Science Center, Fort Collins CO
| Nitrate concentrations at the outlet to Loch
Vale watershed maintained average concentrations of ~1.0 mg/L from 1982
through 1998. In 1999 concentrations began to increase. Five-year running
mean concentrations were nearly twice as high for the years centered around
2003 and 2004, and remain elevated above the 1982-1998 value. Annual precipitation
has been at or below the longterm Loch Vale mean value of 102 cm/year
since 1998, Mean annual temperatures appear to have increased since 1998,
and N deposition is increasing at a rate of about 2% each year. None of
these trends match the stream chemistry pattern, yet precipitation, temperature,
and N deposition probably all contribute in some way. Since nitrogen is
an essential and often limiting nutrient, biogeochemical processes may
also be important to elucidating stream chemistry. We used structural
equation modeling in an attempt to sort out the causal effects of climate,
deposition, and ecosystem processes on stream chemistry. We found interactions
among nearly all watershed components except lake algae, and no clear
driver for the change in stream nitrate concentrations. It appears both
precipitation and temperature influence forest soil processes that in
turn influence stream chemical composition. Alpine soil processes were
less connected to stream chemistry than forests. In addition to the direct
influence of atmospheric nitrogen deposition, precipitation and temperature
had strong, and different, direct influences on stream nitrate. Climate
variability is therefore influencing watershed biogeochemistry in a way
that complicates our efforts to track the effects of atmospheric nitrogen
deposition trends in Loch Vale.
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